The present invention refers to a hydraulic system for suppling a hydrostatic steering system and more particularly to such a hydraulic supply system which supplies both a steering system and secondary devices and controls delivery to assure it to the steering system.
Such hydraulic systems have been proposed for tractors in combination with load-sensing central hydraulic systems. Such a load-sensing system includes a central variable capacity pump, which assumes the entire task of supplying the fluid, and associated individual consuming devices, such as the steering and other secondary consuming devices, along with separate compensated pressure maintaining values. In each case, the valves adjust the supply pressure necessary for the respective consuming device. The compensated pressure maintaining valves are necessary, particularly since the individual consuming devices usually require different operating pressures and the variable capacity pump, whose supply volume and pressure are controlled via the load report lines, must provide the highest supply pressure required at the time. For this purpose, the load report lines are brought together at a changeover valve which transmits the highest load report line pressure in each case to the pump volume governor, for controlling the variable capacity pump.
To assure that the steering of the tractor remains operable at all times, a priority valve is ordinarily provided. It is constructed as a pressure maintaining valve (shown in Federal Republic of Germany OS No. 27 23 490). It can be arranged in the pressure-fluid line leading from the variable capacity pump to the secondary loads. For priority supply of the steering hydraulic circuit, the priority valve is switched from a regulating position into a switch position. This interrupts the connections with the secondary consuming devices, and then all of the fluid conveyed by the pump is conveyed into the steering hydraulic circuit. This produces a considerable improvement in the reliability of operation of the hydraulic system. However, if a leak should occur in the region of the secondary hydraulic circuit or system, the loss of fluid can become so great that the steering is endangered, since the priority valve is not able to recognize such a defect.
In order to increase the dependability of the steering, it has been proposed to incorporate a steering safety valve in the hydraulic circuit. The valve is actuated either electrically or hydraulically. It disconnects the hydraulic circuit of the secondary consuming devices in the event of the occurrence of the above noted defect, to prevent further loss of fluid.
This safety circuit considerably increases the reliablity of the operation of the steering circuit. However, in the event of a large leak, the loss of fluid via the load report line leading to the secondary circuit may still be so great that the steering is endangered.